The present invention relates to a method for preventing electromigration in a magnetic random access memory (MRAM) composed of wordlines, bitlines which intersect the wordlines and resistors which are provided at the points of intersection between the wordlines and the bitlines. The resistors have resistance values that can be influenced by a magnetic field in such a way that they can be assigned to two logic states xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d in that the wordlines and bitlines associated with a selected resistor simultaneously each transmit, in a programming step, a DC signal and the DC signals together generate the magnetic field.
The structure of a conventional MRAM is composed of wordlines and bitlines which essentially intersect the wordlines perpendicularly. At the points of intersection between the wordlines and the bitlines there are memory cells which are each indicated by a resistor.
The resistor represents the path of a tunnel current which flows between a wordline and a bitline, if there is a voltage difference between the wordline and the bitline. The tunnel current then assumes a larger or smaller value as a function of a magnetic field that is written to the memory cell. In other words, the memory cell can be regarded as a binary resistor that is programmed with a larger or smaller resistance value. The information unit xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d can then be assigned to these two resistance values.
A memory cell is therefore programmed by applying a magnetic field. In order to program a memory cell to a value of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, the magnetic field must exceed certain threshold values. It is to be noted here that the memory cell has a magnetic hysteresis.
A DC current that is flowing in the wordline brings about a magnetic field around the wordline. The direction of the magnetic field is reversed here if the current flows in the reverse direction. The direction of the magnetic field then indicates whether a xe2x80x9c1xe2x80x9d or a xe2x80x9c0xe2x80x9d is written to a memory cell.
It will be assumed that the current in the wordline signifies the information current xe2x80x9c1xe2x80x9d. All the memory cells which are connected to the wordline, that is to say the memory cells with the resistors, have the magnetic field applied to them by the current. The effect of the hysteresis of the memory cells with the resistors is then that the magnetic field alone is from a xe2x80x9c0xe2x80x9d state into a xe2x80x9c1xe2x80x9d state. Instead, it is additionally necessary to actuate the bitline, with a current in order to generate, at the interface of the bitline with the wordline, a magnetic field which, as a result of the superposition of the magnetic fields generated by the currents, is sufficiently strong to change the xe2x80x9c0xe2x80x9d state into a xe2x80x9c1xe2x80x9d state at the interface, that is to say at the resistor. In other words, by actuating selected wordlines and selected bitlines it is possible to program the memory cells at the interfaces of these wordlines and bitlines into a xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d state as a function of the direction of the magnetic field generated by the respective currents.
Because currents which are applied in order to program the memory cells are DC currents, electromigration occurs owing to the programming if the same signal, that is to say for example a xe2x80x9c1xe2x80x9d, is for the most part always written to the memory cell. The electromigration by the storage currents in the mA range ultimately signifies that material is transported, which can even lead to the respective metallic leads being destroyed. The service life of MRAMs is therefore undesirably restricted.
It is known that virtually no electromigration occurs when an alternating current is applied. However, it is not possible to use an alternating current in MRAMs because it is not possible to define memory contents by an alternating current.
It is accordingly an object of the invention to provide a method for preventing electromigration in an MRAM which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which prevents electromigration in the MRAM in order to prevent the leads of the MRAM from being destroyed.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for preventing electromigration in a magnetic random access memory (MRAM) containing wordlines, bitlines intersecting the wordlines, and resistors provided at points of intersection between the wordlines and the bitlines. The resistors each have a resistance value that can be influenced by a magnetic field in such a way that it is assigned to two logic states xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. The method includes the steps of simultaneously transmitting, over a wordline and a bitline associated with a selected resistor, in a programming step, DC signals for generating the magnetic field. Each of the wordline and the bitline selected provides a respective DC signal forming the DC signals. Subsequently, transmitting over the wordline and the bitline to which the DC signals have been applied, further DC signals which are of opposite polarity to the DC signals.
The object is achieved according to the invention with a method of the type mentioned at the beginning, in such a way that, after the programming step, the wordline and bitline to which the DC signal is applied transmit a further DC signal which is of opposite polarity to the DC signal. The further DC signal is fed to the wordline and bitline associated with the selected resistor, but with offset timing or with a reduced amplitude and longer time period in comparison with the DC signal.
As a result of the offset in the timing of the further DC signals in the wordline and bitline and the reduced amplitude of the further DC signal, the magnetic field at the interfaces of the wordline and bitline, that is to say in the memory cells, does not exceed the threshold value which leads to information being written to the cell, and thus to the destruction of the information which has already been written.
In accordance with an added mode of the invention, there is the step of feeding the further DC signals with offset timing to the wordline and the bitline associated with the selected resistor.
In accordance with an additional mode of the invention, there is the step of feeding the further DC signals with a reduced amplitude and a longer time period in comparison with the DC signals applied to the wordline and the bitline associated with the selected resistor.
In accordance with a concomitant mode of the invention, there is the step of forming the further DC signals with approximately half an amplitude and twice the time period of the DC signals.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for preventing electromigration in an MRAM, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.